The soil microbe Bacillus thuringiensis (B.t.) is a Gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These often appear microscopically as distinctively shaped crystals. The proteins are highly toxic to pests and specific in their activity. Certain B.t. toxin genes have been isolated and sequenced, and recombinant DNA-based B.t. products produced and approved. In addition, with the use of genetic engineering techniques, new approaches for delivering B.t. endotoxins to agricultural environments are under development, including the use of plants genetically engineered with endotoxin genes for insect resistance and the use of stabilized intact microbial cells as B.t. endotoxin delivery vehicles (Gaertner, F. H., L. Kim [1988] TIBTECH 6:S4-S7). Thus, isolated B.t. endotoxin genes are becoming commercially valuable.
Bacillus thuringiensis produces a proteinaceous paraspore or crystal which is toxic upon ingestion by a susceptible insect host. Over most of the past 30 years, commercial use of B.t. pesticides has been largely restricted to a narrow range of lepidopteran (caterpillar) pests. Preparations of the spores and crystals of B. thuringiensis subsp. kurstaki have been used for many years as commercial insecticides for lepidopteran pests. For example, B. thuringiensis var. kurstaki HD-1 produces a crystal called a delta endotoxin which is toxic to the larvae of a number of lepidopteran insects.
In recent years, however, investigators have discovered B.t. pesticides with specificities for a much broader range of pests. For example, other species of B.t., namely israelensis and san diego (a.k.a. B.t. tenebrionis), have been used commercially to control insects of the orders Diptera and Coleoptera, respectively (Gaertner, F. H. [1989] "Cellular Delivery Systems for Insecticidal Proteins: Living and Non-Living Microorganisms," in Controlled Delivery of Crop Protection Agents, R. M. Wilkins, ed., Taylor and Francis, New York and London, 1990, pp. 245-255). See also Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensi," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, A., A. M. Huger, G. A. Langenbruch, W. Schnetter (1983) Z. ang. Ent. 96:500-508, describe a B.t. isolate named Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
Recently, many new subspecies of B.t. have been identified, and many genes responsible for active .delta.-endotoxin proteins have been isolated (Hofte, H., H. R. Whiteley [1989]Microbiological Reviews 52(2):242-255). Hofte and Whiteley classified 42 B.t. crystal protein genes into 14 distinct genes, grouped into 4 major classes based on amino-acid sequence and host range. The classes were CryI (Lepidoptera-specific), CryII (Lepidoptera-and Diptera-specific), CryIII (Coleoptera-specific), and CryIV (Diptera-specific). The discovery of strains specifically toxic to protozoan pathogens, animal-parasitic liver flukes (Trematoda), or mites (Acari) has broadened the potential B.t. product spectrum even further (see Feitelson, J. S., J. Payne, L. Kim [1992]Bio/Technology 10:271-275). With activities against unique targets, these novel strains retain their very high biological specificity; nontarget organisms remain unaffected. The availability of a large number of diverse B.t. toxins may also enable farmers to adopt product-use strategies that minimize the risk that B.t.-resistant pests will arise.
The cloning and expression of a B.t. crystal protein gene in Escherichia coli has been described in the published literature (see, for example, Schnepf, H.E., H. R. Whitely [1981] Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. No. 4,448,885 and U.S. Pat. No. 4,467,036 both disclose the expression of B.t. crystal protein in E. coli. U.S. Pat. No. 4,853,331 discloses B. thuringiensis strain san diego (a.k.a. B.t. tenebrionis) which can be used to control coleopteran pests in various environments.